The present invention relates to die casting machines, and in particular to a prime mover for hot chamber die casting processes and the like.
At least two different types of die casting machines are presently being used in industry. A first type of die casting machine is referred to as a "cold chamber" machine, and comprises a molten metal reservoir, which is separated from the casting machine, and wherein just enough metal for one casting is ladled by hand into a small chamber, from which it is forced into the die under high pressure. Cold chamber casting machines are generally used in forming aluminum, brass, magnesium, and related alloys. A second type of die casting machine is referred to as a "hot chamber" casting machine, and comprises a basin holding molten metal, a metallic mold or die, and a metal-transferring device which automatically withdraws molten metal from the basin and forces it under pressure into the die. Hot chamber casting machines are typically used in forming zinc, and various zinc alloys.
In one class of hot chamber die casting machines, a plunger is mounted in the basin in which molten metal is retained and is reciprocated by a motor or prime mover to inject the die cavity with molten metal. Typically, the prime mover for the plunger comprises a hydraulic cylinder connected by long hydraulic supply lines with an accumulator that provides a source of high pressure hydraulic fluid. The stroke of the plunger, generally referred to as the "shot stroke," is relatively short in comparison to cold chamber die casting machines, and commences rather slowly past an inlet port for the molten metal, and then accelerates rapidly to a very high speed until the die cavity is completely filled, at which time the back pressure of the injected molten metal suddenly stops extension of the injection plunger.
When a hydraulic cylinder is used as the prime mover for the plunger, the inherent mass and momentum of the hydraulic fluid create problems in properly injecting the die cavity. These problems are particularly prevalent at the beginning and at the end of the shot stroke, when the hydraulic fluid in the prime mover cylinder must be quickly accelerated and decelerated. In accelerating the plunger, a portion of the driving force must be expended to accelerate the hydraulic fluid in the prime mover, and also overcome the frictional forces created by the speed and viscosity of the hydraulic fluid. The frictional losses associated with the fast flowing hydraulic fluid increase dramatically when the plunger is accelerated to the high speed portion of the shot stroke, as the Reynolds number associated with the flow increases exponentially in the range of turbulent flow. The long hydraulic supply lines which are usually required to connect the accumulator with the prime mover exacerbate these problems, and result in severe pressure differentials along the hydraulic lines.
When the piston is decelerated, the kinetic energy of the hydraulic fluid must be quickly dissipated, or the hydraulic fluid will exert an impact force on the injection plunger, which will cause the die to spit. Further, this type of impact force on the injection plunger will cause the plunger to bounce or rebound back from the bottom dead center position, which creates a recoil or backlash, comprising a negative velocity spike that results in the formation of cavities in the casting and ruins their integrity. A hydraulic hammering effect can also be experienced due to the celerity of the resulting impact wave relative to the velocity of hydraulic fluid in the prime mover system.